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<h1><img src="../../../boost.png" alt="boost.png (6897 bytes)" align=
"middle" width="277" height="86">Boost.MultiIndex Future work</h1>

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<p>
A number of new functionalities are considered for inclusion into
future releases of Boost.MultiIndex. Some of them depend on the
potential for extensibility of the library, which has been a guiding
principle driving the current internal design of <code>multi_index_container</code>.
</p>

<h2>Contents</h2>

<ul>
  <li><a href="#notifying">Notifying indices</a></li>
  <li><a href="#constraints">Constraints</a></li>
  <li><a href="#user_defined_indices">User-defined indices</a></li>
  <li><a href="#indexed_maps">Indexed maps</a></li>
</ul>

<h2><a name="notifying">Notifying indices</a></h2>

<p>
<i>Notifying indices</i> can be implemented as decorators over
preexistent index types, with the added functionality that internal
events of the index (insertion, erasing, modifying of elements) are
signalled to an external entity --for instance, by means of the
<a href="../../../doc/html/signals.html">Boost.Signals</a>
library. This functionality can have applications for:
<ol>
  <li>Logging,</li>
  <li>interfacing to GUI-based applications,</li>
  <li>synchronization between separate data structures.</li>
</ol>
</p>

<p>
The following is a sketch of a possible realization of notifying
indices:
</p>

<blockquote><pre>
<span class=keyword>struct</span> <span class=identifier>insert_log</span>
<span class=special>{</span>
  <span class=keyword>void</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>)</span>
  <span class=special>{</span>
    <span class=identifier>std</span><span class=special>::</span><span class=identifier>clog</span><span class=special>&lt;&lt;</span><span class=string>&quot;insert: &quot;</span><span class=special>&lt;&lt;</span><span class=identifier>x</span><span class=special>&lt;&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>endl</span><span class=special>;</span>
  <span class=special>}</span>
<span class=special>};</span>

<span class=keyword>int</span> <span class=identifier>main</span><span class=special>()</span>
<span class=special>{</span>
  <span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
    <span class=keyword>int</span><span class=special>,</span>
    <span class=identifier>indexed_by</span><span class=special>&lt;</span>
      <span class=identifier>notifying</span><span class=special>&lt;</span><span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=special>&gt;,</span> <span class=comment>// notifying index</span>
      <span class=identifier>ordered_non_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;</span>
    <span class=special>&gt;</span>
  <span class=special>&gt;</span> <span class=identifier>indexed_t</span><span class=special>;</span>

  <span class=identifier>indexed_t</span> <span class=identifier>t</span><span class=special>;</span>

  <span class=comment>// on_insert is the signal associated to insertions</span>
  <span class=identifier>t</span><span class=special>.</span><span class=identifier>on_insert</span><span class=special>.</span><span class=identifier>connect</span><span class=special>(</span><span class=identifier>insert_log</span><span class=special>());</span>

  <span class=identifier>t</span><span class=special>.</span><span class=identifier>insert</span><span class=special>(</span><span class=number>0</span><span class=special>);</span>
  <span class=identifier>t</span><span class=special>.</span><span class=identifier>insert</span><span class=special>(</span><span class=number>1</span><span class=special>);</span>

  <span class=keyword>return</span> <span class=number>0</span><span class=special>;</span>
<span class=special>}</span>

<span class=comment>// output:
//   insert: 0
//   insert: 1</span>
</pre></blockquote>

<h2><a name="constraints">Constraints</a></h2>

<p>
The notifying indices functionality described above exploits a powerful
design pattern based on <i>index adaptors</i>, decorators over preexistent
indices which add some functionality or somehow change the semantics of
the underlying index. This pattern can be used for the implementation
of <i>constraints</i>, adaptors that restrict the elements accepted by an
index according to some validation predicate. The following is a possible
realization of how constraints syntax may look like: 
</p>

<blockquote><pre>
<span class=keyword>struct</span> <span class=identifier>is_even</span>
<span class=special>{</span>
  <span class=keyword>bool</span> <span class=keyword>operator</span><span class=special>()(</span><span class=keyword>int</span> <span class=identifier>x</span><span class=special>)</span><span class=keyword>const</span><span class=special>{</span><span class=keyword>return</span> <span class=identifier>x</span><span class=special>%</span><span class=number>2</span><span class=special>==</span><span class=number>0</span><span class=special>;}</span>
<span class=special>};</span>

<span class=keyword>typedef</span> <span class=identifier>multi_index_container</span><span class=special>&lt;</span>
  <span class=keyword>int</span><span class=special>,</span>
  <span class=identifier>indexed_by</span><span class=special>&lt;</span>
    <span class=identifier>constrained</span><span class=special>&lt;</span><span class=identifier>ordered_unique</span><span class=special>&lt;</span><span class=identifier>identity</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;</span> <span class=special>&gt;,</span><span class=identifier>is_even</span><span class=special>&gt;</span>
  <span class=special>&gt;</span>
<span class=special>&gt;</span> <span class=identifier>indexed_t</span><span class=special>;</span>
</pre></blockquote>

<h2><a name="user_defined_indices">User-defined indices</a></h2>

<p>
The mechanisms by which Boost.MultiIndex orchestrates the
operations of the indices held by a <code>multi_index_container</code> are
simple enough to make them worth documenting so that the (bold)
user can write implementations for her own indices.
</p>

<h2><a name="indexed_maps">Indexed maps</a></h2>

<p>
<code>multi_index_container</code> is rich enough to provide the basis
for implementation of <i>indexed maps</i>, i.e. maps which
can be looked upon several different keys. The motivation for having
such a container is mainly aesthetic convenience, since it
would not provide any additional feature to similar constructs
based directly on <code>multi_index_container</code>.
</p>

<p>
The main challenge in writing an indexed map lies in the design of a
reasonable interface that resembles that of <code>std::map</code> as
much as possible. There seem to be fundamental difficulties in extending
the syntax of a <code>std::map</code> to multiple keys. For one example,
consider the situation:
</p>

<blockquote><pre>
<span class=identifier>indexed_map</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>,</span><span class=identifier>string</span><span class=special>,</span><span class=keyword>double</span><span class=special>&gt;</span> <span class=identifier>m</span><span class=special>;</span>
<span class=comment>// keys are int and string, double is the mapped to value</span>

<span class=special>...</span>

<span class=identifier>cout</span><span class=special>&lt;&lt;</span><span class=identifier>m</span><span class=special>[</span><span class=number>0</span><span class=special>]&lt;&lt;</span><span class=identifier>endl</span><span class=special>;</span>      <span class=comment>// OK</span>
<span class=identifier>cout</span><span class=special>&lt;&lt;</span><span class=identifier>m</span><span class=special>[</span><span class=string>&quot;zero&quot;</span><span class=special>]&lt;&lt;</span><span class=identifier>endl</span><span class=special>;</span> <span class=comment>// OK</span>
<span class=identifier>m</span><span class=special>[</span><span class=number>1</span><span class=special>]=</span><span class=number>1.0</span><span class=special>;</span>              <span class=comment>// !!</span>
</pre></blockquote>

<p>
In the last sentence of the example, the user has no way of
providing the <code>string</code> key mapping to the same value
as <code>m[1]</code>. This and similar problems have to be devoted
a careful study when designing the interface of a potential
indexed map.
</p>

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<br>

<p>Revised April 19th 2015</p>

<p>&copy; Copyright 2003-2015 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
Distributed under the Boost Software 
License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt">
LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
http://www.boost.org/LICENSE_1_0.txt</a>)
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